Hot-rolled steel sheet having excellent cold workability and excellent surface properties and hardness after working

ABSTRACT

A hot-rolled steel sheet which has a thickness of 3-20 mm and contains specific amounts of C, Si, Mn, P, S, Al and N, with the balance made up of iron and unavoidable impurities. This hot-rolled steel sheet contains a specific amount of solid-solved N, and the contents of C and N satisfy the relation 10C+N≦3.0. This hot-rolled steel sheet contains bainitic ferrite and pearlite, respectively in an area ratio of 5% or more and in an area ratio of less than 20% relative to the whole structure, with the balance made up of polygonal ferrite. The average crystal grain size of the bainitic ferrite is 3-50 μm.

TECHNICAL FIELD

The present invention relates to a hot-rolled steel sheet exhibiting good cold workability during working and exhibiting, after working, predetermined surface property (sometimes referred to as “surface quality”) and post-working hardness.

BACKGROUND ART

In recent years, from the standpoint of environmental protection, lighter weight, i.e., higher strength, of steel materials for use in various parts for automotive, for example, transmission parts such as gear, and casings, is increasingly required with the purpose of enhancing the fuel efficiency of automobiles. To meet this requirement for lighter weight and higher strength, a steel material prepared by hot-forging a steel bar (hot-forged material) has been used as a commonly-employed steel material. In addition, in order to reduce CO₂ emission in the process of producing parts, a requirement for cold forging of parts such as a gear, which had been heretofore worked by hot forging, is also more and more increasing.

Cold working (cold forging) is advantageous in that the productivity is high compared with hot working and warm working and moreover, both the dimensional accuracy and the steel material yield are good. The problem occurring in the case of producing parts by the cold working is that a steel material having high strength, i.e., high deformation resistance, must be necessarily used so as to ensure that the strength of cold-worked parts is equal to or more than a predetermined value expected. However, a higher deformation resistance of a steel material used leads to a shortening of the life of a metal mold for cold working.

In the field of transmission parts, studies on the production of parts using a steel sheet instead of a forged product (hot forging, cold forging, etc.) of a steel bar are ongoing with an aim to reduce the weight and cost of the parts, but there is a drawback that in the parts after cold working of a steel sheet, a surface defect called stretcher strain mark (hereinafter, simply referred to as “SS mark”) is readily generated on the surface.

Therefore, conventionally, a method where a steel material is cold-forged into a predetermined shape and then subjected to a heat treatment such as quenching-tempering to produce a high-strength part assured of predetermined strength (hardness) is sometimes conducted. However, the heat treatment after cold forging inevitably causes a change in part dimension and therefore, it must be secondarily corrected by machining such as cutting. A possible solution to omit the step of heat treatment or subsequent working has been demanded.

In order to solve the problems above, for example, it is disclosed that when the progress of natural aging is restrained by using solute C in a low-carbon steel to ensure a predetermined amount of age hardening due to strain aging, a wire rod/steel bar for cold forging, excellent in the strain aging property, can be obtained (see, Patent Document 1).

However, in this technique, strain aging is controlled only by the solute C amount, and a steel material having sufficient cold workability as well as predetermined surface quality and hardness/strength after working can be hardly obtained.

Then, the present applicant had made various studies by focusing the attention on the difference of the effects of solute C and solute N contained in a steel material on the deformation resistance and static strain aging. As a result, it was found that when the amounts of these solute elements are appropriately controlled, a steel material for mechanical structure exerting good cold workability during working and exhibiting predetermined hardness (strength) after cold working (cold forging) can be obtained. The present applicant has already filed a patent application based on this finding (see, Patent Document 2).

This steel material realizes both cold workability and higher hardness (higher strength) after working but is a hot-forged material, similarly to the wire rod/steel bar described in Patent Document 1, and the production cost is disadvantageously high. In order to more reduce the production cost, studies are also being made to produce automobile parts by cold working by using a hot-rolled steel sheet in place of the conventional hot-forged material.

For example, a hot-rolled steel sheet for nitriding treatment, ensuring that high surface hardness and sufficient hardening depth are obtained after nitriding treatment, has been proposed (see, Patent Document 3).

However, this technique further requires a nitriding treatment after cold working and has a problem that a sufficient cost reduction cannot be realized.

In addition, a hot-rolled steel sheet having a composition containing C: 0.10% or less, Si: less than 0.01%, Mn: 1.5% or less, and Al: 0.20% or less, containing (Ti+Nb)/2: from 0.05 to 0.50%, and containing S: 0.005% or less, N: 0.005% or less, and O: 0.004% or less, with the total of S, N and O being 0.0100% or less, where the microstructure is a substantially ferrite single phase of 95% or more, has been proposed. This hot-rolled steel sheet is thought to be excellent in the dimensional accuracy of a finely blanked surface, ensure very high surface hardness of the blanked surface after working, and also be excellent in the resistance to red-scale defect (see, Patent Document 4).

However, this hot-rolled steel sheet where N is limited to a very low content as a harmful element utterly differs in the technical idea from the hot-rolled steel sheet according to the present invention where N is positively utilized.

RELATED ART Patent Document

Patent Document 1: JP-A-10-306345

Patent Document 2: JP-A-2009-228125

Patent Document 3: JP-A-2007-162138

Patent Document 4: JP-A-2004-137607

SUMMARY OF THE INVENTION Problems that the Invention is to Solve

By taking notice of these circumstances, the present invention has been made, and an object thereof is to provide a hot-rolled steel sheet exhibiting good cold workability during working and exhibiting predetermined surface property and hardness after working.

Means for Solving the Problems

The invention described in claim 1 is a hot-rolled steel sheet excellent in cold workability as well as in surface property and hardness after working, in which:

a sheet thickness is from 3 to 20 mm;

a component composition contains, in mass% (hereinafter, the same applies to chemical components),

C: 0.3% or less (exclusive of 0%),

Si: 0.5% or less (exclusive of 0%),

Mn: from 0.2 to 1%,

P: 0.05% or less (exclusive of 0%),

S: 0.05% or less (exclusive of 0%),

Al: from 0.01 to 0.1%, and

N: from 0.008 to 0.025%,

with a remainder being iron and unavoidable impurities, in which solute N: 0.007% or more and

the contents of C and N satisfy the relationship of 10C+N≦3.0;

a microstructure contains, in terms of area ratio relative to an entire microstructure,

bainitic ferrite: 5% or more,

pearlite: less than 20%, and

remainder: polygonal ferrite; and

an average grain size of the bainitic ferrite is in a range of from 3 to 50 μm.

The invention described in claim 2 is the hot-rolled steel sheet according to claim 1, in which the component composition further contains at least one member of the following (a) to (e):

-   -   (a) at least one of Cr: 2% or less (exclusive of 0%) and Mo: 2%         or less (exclusive of 0%),     -   (b) at least one member selected from the group consisting of         Ti: 0.2% or less (exclusive of 0%), Nb: 0.2% or less (exclusive         of 0%) and V: 0.2% or less (exclusive of 0%),     -   (c) B: 0.005% or less (exclusive of 0%),     -   (d) at least one member selected from the group consisting of         Cu: 5% or less (exclusive of 0%), Ni: 5% or less (exclusive of         0%) and Co: 5% or less (exclusive of 0%), and     -   (e) at least one member selected from the group consisting of         Ca: 0.05% or less (exclusive of 0%), REM: 0.05% or less         (exclusive of 0%), Mg: 0.02% or less (exclusive of 0%), Li:         0.02% or less (exclusive of 0%), Pb: 0.5% or less (exclusive of         0%), and Bi: 0.5% or less (exclusive of 0%).

Advantage of the Invention

According to the present invention, in a microstructure mainly composed of bainitic ferrite having a predetermined average grain size +polygonal ferrite, the solute N amount is ensured and at the same time, the C content and the N content satisfy a predetermined relationship, so that a hot-rolled steel sheet which is reduced in the deformation resistance during cold working, thereby not only extending the life of a metal mold but also hardly allowing occurrence of cracking in a steel sheet, and can assure the parts obtained after working of predetermined surface property and post-working hardness, can be provided.

MODE FOR CARRYING OUT THE INVENTION

The hot-rolled steel sheet according to the present invention (hereinafter, sometimes referred to as “steel sheet of the present invention” or simply as “steel sheet”) is described in more detail below. The steel sheet of the present invention is common with the hot-forged material described in Patent Document 2 in that the amount of N dissolved in solid is ensured and at the same time, the C content and the N content satisfy a predetermined relationship, but differs in that the C content is allowable up to a somewhat high range, the microstructure is a bainitic ferrite-polygonal ferrite-pearlite multi-phase microstructure and at the same time, the bainitic ferrite particle is refined.

[Thickness of Steel Sheet of the Present Invention: from 3 to 20 mm]

First, a steel sheet having a thickness of 3 to 20 mm is targeted by the steel sheet of the present invention. If the sheet thickness is less than 3 mm, the rigidity as a structure cannot be ensured. On the other hand, if the sheet thickness exceeds 20 mm, the microstructure specified in the present invention can be hardly achieved, and the desired effects cannot be obtained. The sheet thickness is preferably from 4 to 19 mm.

Next, the component composition constituting the steel sheet of the present invention is described. In the following, the units of chemical components are all mass %.

[Component Composition of Steel Sheet of the Present Invention] <C: 0.3% or Less (Exclusive of 0%)>

C is an element greatly affecting the formation of the microstructure of the steel sheet and although the microstructure is a bainitic ferrite-polygonal ferrite-pearlite multi-phase microstructure, in order to form a bainitic ferrite-polygonal ferrite-based microstructure with as little pearlite as possible, the content of this element needs to be limited. If C is contained too much, the pearlite fraction in the steel sheet microstructure is increased, leaving a fear that the deformation resistance becomes excessive due to work hardening of pearlite. Therefore, the C content in the steel sheet is limited to 0.3% or less, preferably 0.25% or less, more preferably 0.2% or less, and still more preferably 0.15% or less. However, if the C content is too small, deoxidation during melting of steel is difficult to be achieved and at the same time, the strength and hardness after cold working can be hardly satisfied. Therefore, it is preferably 0.0005% or more, more preferably 0.0008% or more and still more preferably 0.001% or more.

<Si: 0.5% or Less (Exclusive of 0%)>

Si forms a solid solution in steel to thereby increase the deformation resistance of the steel sheet and thus, is an element needed to be reduced as much as possible. Therefore, in order to suppress the increase in deformation resistance, the Si content in the steel sheet is limited to 0.5% or less, preferably 0.45% or less, more preferably 0.4% or less, and still more preferably 0.3% or less. However, if the Si content is extremely small, deoxidation during melting is difficult to be achieved and at the same time, the strength and hardness after cold working can be hardly satisfied. Therefore, it is preferably 0.005% or more, more preferably 0.008% or more, and still more preferably 0.01% or more.

<Mn: from 0.2 to 1%>

Mn is an element exerting deoxidation and desulfurization actions in the process of steel making. Furthermore, when the N content in the steel material is increased, cracking is readily generated due to dynamic strain aging by heat generation during working, but, on the other hand, Mn has an effect of enhancing the workability on this occasion and inhibiting cracking. In order to effectively bring out these actions, the Mn content in the steel sheet is 0.2% or more, preferably 0.22% or more and more preferably 0.25% or more. However, if the Mn content is too large, the deformation resistance becomes excessive, and segregation occurs to produce a heterogeneity in the microstructure. Therefore, it is 1% or less, preferably 0.98% or less and more preferably 0.95% or less.

<P: 0.05% or Less (Exclusive of 0%)>

P is an impurity element unavoidably contained in the steel. It is an element that, if contained in ferrite, segregates at a ferrite grain boundary to deteriorate cold workability and contributes to solid-solution hardening of ferrite and thereby gives rise to an increase in the deformation resistance. Therefore, the P content is preferably reduced as much as possible in view of cold workability, but if excessively reduced, the steel making cost increases. Therefore, in consideration of process capability, the content is 0.05% or less and preferably 0.03% or less.

<S: 0.05% or Less (Exclusive of 0%)>

S is also an unavoidable impurity, similarly to P, and is an element precipitating as FeS at a grain boundary in a film form to deteriorate the workability. In addition, this also has an action of causing hot shortness. In the present invention, from the standpoint of enhancing the deformation performance, the S content is 0.05% or less and preferably 0.03% or less. However, reduction of the S content to 0 is difficult in industry. Since S has an effect of enhancing the machinability, in view of machinability enhancement, it is recommended to be contained in an amount of preferably 0.002% or more and more preferably 0.006% or more.

<Al: from 0.01 to 0.1%>

Al is an element effective for deoxidation in the process of steel making. In order to obtain this deoxidation effect, the Al content in the steel sheet is 0.01% or more, preferably 0.015% or more and more preferably 0.02% or more. However, if the Al content is excessively large, toughness is reduced and cracking readily occurs. Therefore, the content is 0.1% or less, preferably 0.09% or less and more preferably 0.08% or less.

<N: from 0.008 to 0.025%>

N is an important element for obtaining predetermined strength by static strain aging after working. Therefore, the N content in the steel sheet is 0.008% or more, preferably 0.0085% or more and more preferably 0.009% or more. However, if the N content is excessively large, the effect of dynamic strain aging during working, in addition to static strain aging, becomes significant, and thus the deformation resistance is increased, which is unsuitable. Therefore, the content is 0.025% or less, preferably 0.023% or less and more preferably 0.02% or less.

<Solute N: 0.007% or More>

When a predetermined amount of solute N (hereinafter, referred to as “solute N amount”) is ensured in the steel sheet, the static strain aging can be promoted without increasing the deformation resistance very much. In order to ensure predetermined strength after cold working, the solute N amount needs to be 0.007% or more. However, if the solute N amount is excessively large, not only the cold workability is deteriorated but also the amount of the solute N fixed to working strain is increased, and as a result, SS mark is readily generated and the surface property is deteriorated as well. Therefore, it is preferably 0.03% or less. In this connection, since the N content in the steel material is 0.025% or less, the solute N amount is substantially kept from becoming 0.025% or more.

Here, the solute N amount in the present invention is an amount determined by subtracting the amount of total N compounds from the total N amount in the steel sheet in conformity with JIS G 1228. An example of the practical method for measuring the solute N amount is described below.

(a) Inert Gas Fusion Method-Thermal Conductivity Method (Measurement of Total N Amount)

A sample cut out from a test material is placed in a crucible and fused in an inert gas stream to extract N, and the extract is transferred to a thermal conductivity cell and measured for the change in thermal conductivity to determine the total N amount. (b) Ammonia Distillative Separation and Indophenol Blue Absorptiometry (measurement of amount of total N compounds)

A sample cut out from a test material is dissolved in a 10% AA-type electrolytic solution and a constant current electrolysis is performed to measure the amount of total N compounds in the steel. The 10% AA-type electrolytic solution used is a nonaqueous solvent-type electrolytic solution composed of 10% acetone and 10% tetramethylammonium chloride, with the remainder being methanol, and is a solution not forming a passive film on the steel surface.

About 0.5 g of the sample of the test material is dissolved in the 10% AA-type electrolytic solution, and the insoluble residue (N compounds) produced is filtered through a polycarbonate-made filter having a pore size of 0.1 μm. The obtained insoluble residue is decomposed by heating in sulfuric acid, potassium sulfate and pure copper-made chips, and the decomposition product is combined with the filtrate. The resulting solution is made alkaline with sodium hydroxide and then subjected to steam distillation, and the distilled ammonia is absorbed by diluted sulfuric acid. Furthermore, phenol, sodium hypochlorite and sodium pentacyanonitrosylferrate(III) are added to produce a blue complex, and the absorbance thereof is measured by using an absorptiometer to determine the amount of total N compounds.

The solute N amount can be determined by subtracting the amount of total N compounds determined by the method (b) from the total N amount determined by the method (a).

<Contents of C and N Satisfy the Relationship of 10C+N≦3.0>

In the steel material of the present invention, solute C greatly increases the deformation resistance and does not so much contribute to static strain aging and, on the other hand, the solute N can promote the static strain aging without raising the deformation resistance very much and therefore, has an action of allowing for an increase in the hardness after working. Therefore, in the steel material of the present invention, in order to increase the hardness after working without raising the deformation resistance during working very much, the C content and the N content must satisfy the relationship of 10C+N≦3.0. It is preferably 0.009≦10C+N≦2.8, more preferably 0.01≦10C+N≦2.5 and still more preferably 0.01≦10C+N≦2.0. From the standpoint of refining the grain in the hot-rolled steel sheet and ensuring formability of the steel sheet, the C content and the solute C amount are needed to some extent, but if 10C+N>3.0, the amounts of C and/or N are too large, and the deformation resistance becomes excessive. In the inequality above, the coefficient of the C content is set to be 10 times the coefficient of the N content by taking into account the fact that even when the contents are the same, the degree of increase in the strength and deformation resistance in the hot-rolled steel sheet of the present invention, which is attributable to the solute C, is about one digit (10 times) larger than that attributable to the solute N.

The steel of the present invention fundamentally contains the above-described components, with the remainder being iron and unavoidable impurities, but in addition, the following allowable components may be added, as long as the action of the present invention is not impaired.

<Cr: 2% or Less Eexclusive of 0%) and/or Mo: 2% or Less (Exclusive of 0%)>

Cr is an element having an action of increasing the grain boundary strength and thereby enhancing the deformation performance of the steel. In order to effectively bring out such an action, Cr is preferably contained in an amount of 0.2% or more, but if Cr is contained too much, the deformation resistance may be increased to reduce the cold workability. Therefore, it is recommended that the content thereof is 2% or less, furthermore 1.5% or less, and in particular 1% or less.

Mo is an element having an action of increasing the hardness of the steel material after working and the deformation performance. In order to effectively bring out such an action, Mo is preferably contained in an amount of 0.04% or more, more preferably 0.08% or more. However, if Mo is contained too much, the cold workability may be deteriorated. Therefore, it is recommended that the content thereof is 2% or less, furthermore 1.5% or less, and in particular 1% or less.

<At Least One Member Selected from the Group Consisting of Ti: 0.2% or Less (Exclusive of 0%), Nb: 0.2% or Less (Exclusive of 0%) and V: 0.2% or Less (Exclusive of 0%)>

These elements have a high affinity for N and are elements fulfilling the role of forming N compounds by coexisting with N, refining the grain of steel, enhancing the toughness of a processed product obtained after cold working, and also enhancing the cracking resistance. Even if each element is contained in an amount over the upper limit value, an effect of improving the property is not obtained. Therefore, it is recommended that the content of each element is 0.2% or less, furthermore from 0.001 to 0.15% and in particular from 0.002 to 0.1%.

<B: 0.005% or Less (Exclusive of 0%)>

Similarly to Ti, Nb and V above, B has a high affinity for N and is an element fulfilling the role of forming a N compound by coexisting with N, refining the grain of steel, enhancing the toughness of a processed product obtained after cold working, and also enhancing the cracking resistance. Therefore, in the case where the steel sheet of the present invention contains B, a predetermined solute N amount can be ensured to enhance the strength after cold working. For this reason, it is recommended that the content thereof is 0.005% or less, furthermore from 0.0001 to 0.0035% and in particular from 0.0002 to 0.002%.

<At Least One Member Selected from the Group Consisting of Cu: 5% or Less (Exclusive of 0%), Ni: 5% or Less (Exclusive of 0%) and Co: 5% or Less (Exclusive of 0%)>

All of these elements have an action of hardening the steel material by stain aging and are elements effective for enhancing the post-working strength. In order to effectively bring out such an action, each of these elements is preferably contained in an amount of 0.1% or more and furthermore 0.3% or more. However, if the content of each of these elements is too much large, the effect of hardening the steel material by stain aging and furthermore the effect of enhancing the post-working strength may be saturated, or the cracking may be promoted. Therefore, it is recommended that each of them is 5% or less, furthermore 4% or less and in particular 3% or less.

<At Least One Member Selected from the Group Consisting of Ca: 0.05% or Less (Exclusive of 0%), REM: 0.05% or Less (Exclusive of 0%), Mg: 0.02% or Less (Exclusive of 0%), Li: 0.02% or Less (Exclusive of 0%), Pb: 0.5% or Less (Exclusive of 0%), and Bi: 0.5% or Less (Exclusive of 0%)>

Ca is an element spheroidizing a sulfide compound-based inclusion such as MnS to thereby enhance the deformation performance of steel and at the same time, contributing to improvement of the machinability. In order to effectively bring out such an action, Ca is preferably contained in an amount of 0.0005% or more and furthermore 0.001% or more. Even if contained too much, the effect thereof is saturated and an effect consistent with the content cannot be expected. Therefore, 0.05% or less, furthermore 0.03% or less and in particular 0.01% or less are recommended.

REM is, similarly to Ca, an element spheroidizing a sulfide compound-based inclusion such as MnS to thereby enhance the deformation performance of steel and at the same time, contributing to improvement of the machinability. In order to effectively bring out such an action, REM is preferably contained in an amount of 0.0005% or more and furthermore 0.001% or more. Even if contained too much, the effect thereof is saturated and an effect consistent with the content cannot be expected. Therefore, 0.05% or less, furthermore 0.03% or less and in particular 0.01% or less are recommended.

The “REM” as used in the present invention means to include lanthanoid elements (15 elements from La to Lu) as well as Sc (scandium) and Y (yttrium). Among these elements, it is preferable to contain at least one element selected from the group consisting of La, Ce and Y, and it is more preferable to contain La and/or Ce.

Mg is, similarly to Ca, an element spheroidizing a sulfide compound-based inclusion such as MnS to thereby enhance the deformation performance of steel and at the same time, contributing to improvement of the machinability. In order to effectively bring out such an action, Mg is preferably contained in an amount of 0.0002% or more and furthermore 0.0005% or more. Even if contained too much, the effect thereof is saturated and an effect consistent with the content cannot be expected. Therefore, 0.02% or less, furthermore 0.015% or less and in particular 0.01% or less are recommended.

Li is, similarly to Ca, an element spheroidizing a sulfide compound-based inclusion such as MnS to allow for enhancement of the deformation performance of steel and in addition, contributing to improvement of the machinability by lowering the melting point of an Al-based oxide and thereby making it harmless. In order to effectively bring out such an action, Li is preferably contained in an amount of 0.0002% or more and furthermore 0.0005% or more. Even if contained too much, the effect thereof is saturated and an effect consistent with the content cannot be expected. Therefore, 0.02% or less, furthermore 0.015% or less and in particular 0.01% or less are recommended.

Pb is an element effective for enhancing the machinability. In order to effectively bring out such an action, Pb is preferably contained in an amount of 0.005% or more and furthermore 0.01% or more. However, if contained too much, there arises a problem with production such as formation of a roll mark. Therefore, 0.5% or less, furthermore 0.4% or less and in particular 0.3% or less are recommended.

Bi is, similarly to Pb, an element effective for enhancing the machinability. In order to effectively bring out such an action, Bi is preferably contained in an amount of 0.005% or more and furthermore 0.01% or more. Even if contained too much, the effect of enhancing the machinability is saturated. Therefore, 0.5% or less, furthermore 0.4% or less and in particular 0.3% or less are recommended.

The microstructure characterizing the steel sheet of the present invention is described below.

[Microstructure of Steel Sheet of the Present Invention]

As described above, the steel sheet of the present invention has the steel of a bainitic ferrite-polygonal ferrite-pearlite multi-phase microstructure as a base and, particularly, is characterized in that the size of the bainitic ferrite particle is controlled to a specific range.

<Bainitic Ferrite: 5% or More, Pearlite: Less Than 20% and Remainder: Polygonal Ferrite>

The microstructure of the steel sheet of the present invention is composed of a multi-phase microstructure of bainitic ferrite, polygonal ferrite and pearlite. Bainitic ferrite has an action of enhancing the workability during cold working while increasing the hardness after working, and at the same time, suppressing generation of a stretcher stain mark. In order to effectively bring out such an action, the content thereof is, in terms of area ratio, 5% or more, preferably 10% or more and more preferably 15% or more. If an excess of pearlite is present, the formability of the steel sheet is deteriorated. Therefore, pearlite is, in terms of area ratio, less than 20%, preferably 19% or less, more preferably 18% or less and still more preferably 15% or less. The remainder is polygonal ferrite.

In addition to the above-described microstructure, a cementite phase is also present in the microstructure of the steel sheet of the present invention, but its amount is as very small as about 1% or less at the most in terms of area ratio. Therefore, in the description of the present invention, respective area ratios of bainitic ferrite, polygonal ferrite and pearlite are defined as ones specified such that the total area ratio of these three phases is 100%.

<Average Grain Size of Bainitic Ferrite: in a Range of from 3 to 50 μm>

The average grain size of bainitic ferrite constituting the bainitic ferrite microstructure must be in a range of from 3 to 50 μm so as to enhance the workability of the steel sheet and satisfy the surface property after working. If the bainitic ferrite particles are excessively fine, the deformation resistance becomes too high. Therefore, the average grain size thereof is 3 μm or more, preferably 4 μm or more and more preferably 5 μm or more. On the other hand, if the bainitic ferrite is excessively coarsened, the surface property after working is deteriorated and in addition, toughness, fatigue property, etc. are reduced. Therefore, the average grain size thereof is 50 μm or less, preferably 45 μm or less and more preferably 40 μm or less.

[Method for Measuring Area Ratio of Each Phase]

As for the area ratio of the each phase above, each test steel sheet is subjected to Nital etching, and five visual fields are photographed by a scanning electron microscope (SEM, magnification: 1,000 times), and as a result, respective percentages of bainitic ferrite, polygonal ferrite and pearlite can be determined by a point counting method.

Here, the bainitic ferrite is defined as a ferrite particle existing in the bainite (collectively referring to upper bainite and lower bainite) microstructure in which the grain is in an axially extended shape (see, Tadashi Furuhara, “Current Opinion on Definition of Bainite Structure in Steels”, Netsu Shori, Vol. 50, No. 1, February 2010, pp. 22-27) and an aspect ratio (major axis/minor axis ratio) is 2 or more. In addition, the polygonal ferrite is defined as a ferrite particle in which the grain is in an equiaxed shape and an aspect ratio (major axis/minor axis ratio) is less than 2.

[Method for Measuring Average Grain Size]

The average grain size of the bainitic ferrite above can be measured as follows.

That is, the grain sizes of bainitic ferrite present at three portions, i.e., an outermost layer part, a part at ¼ of the sheet thickness, and a central part in the sheet thickness direction, are measured. As to the grain size of one bainitic ferrite particle, the side surface part in the rolling direction at each measurement portion is subjected to Nital etching, five visual fields of the corresponding region are photographed by a scanning electron microscope (SEM; magnification: 1,000 times), and the diameter including the center of gravity of the bainitic ferrite grain is determined by image analysis and defined as the average grain size.

A preferable production method for obtaining the above-described steel sheet of the present invention is described below.

[Preferable Method for Producing Steel Sheet of the Present Invention]

The production of the steel sheet of the present invention may be conducted according to any method as long as it is a method capable of forming a raw material steel having the above-described chemical composition into a desired thickness. For example, it can be conducted by a method in which, under following conditions, a molten steel having the above-described component composition is prepared in a converter, subjected to ingot making or continuous casting to form a slab, and then rolled into a hot-rolled steel sheet having a desired thickness.

[Preparation of Molten Steel]

The N content in the molten steel can be adjusted by adding a N compound-containing raw material to the molten steel and/or controlling the atmosphere of the converter to a N₂ atmosphere, during melting in the converter.

[Heating]

Heating before hot rolling is performed at 1,100 to 1,300° C. In this heating, a high-temperature heating condition is necessary so as to produce no N compound and dissolve as much N as possible in solid. As for the heating temperature, preferable lower limit is 1,100° C. and more preferable lower limit is 1,150° C. On the other hand, a temperature more than 1,300° C. is operationally difficult.

[Hot Rolling]

Hot rolling is performed such that the finish rolling temperature is 880° C. or higher. If the finish rolling temperature is too low, ferrite transformation takes place at a high temperature, leading to coarsening of the precipitated carbide in ferrite (collectively referring to bainitic ferrite and polygonal ferrite), and the fatigue strength is deteriorated. Therefore, a finish rolling temperature not less than a certain level is necessary. The finish rolling temperature is more preferably 900° C. or more so as to coarsen the austenite particle and thereby increase the grain size of bainitic ferrite to a certain extent. The upper limit of the finish rolling temperature is 1,000° C. because temperature ensuring is difficult.

The thickness of the hot-rolled steel sheet of the present invention is from 3 to 20 mm. In order to refine the bainitic ferrite grain and thereby control the average grain size thereof to fall in a predetermined grain size range, not only the rolling temperature must be controlled as above but also the final rolling reduction by tandem rolling in the finish rolling must be controlled to be 15% or more. Usually, in the finish rolling, tandem rollings of from 5 to 7 passes are conducted, where the pass schedule is set from the standpoint of controlling jamming of the sheet and the final rolling reduction is up to approximately from 12 to 13%. The final rolling reduction is preferably 16% or more and more preferably 17% or more. As the final rolling reduction is higher, e.g., 20% or 30%, the effect of more refining the grain is obtained, but in view of rolling control, the upper limit is specified to be about 30%.

[Rapid Cooling After Hot Rolling]

After the completion of the finish rolling, the sheet is rapidly cooled at a cooling rate (first cooling rate) of 20° C./s or more within 5 seconds and the rapid cooling is stopped at a temperature (rapid cooling stop temperature) of 550° C. or more and less than 650° C. This is performed so as to obtain a bainitic ferrite-polygonal ferrite-pearlite multi-phase microstructure having predetermined phase fractions. If the cooling rate (rapid cooling rate) is less than 20° C./s, pearlite transformation is promoted, and if the rapid cooling stop temperature is less than 550° C., bainite transformation is suppressed. In both cases, a bainitic ferrite-polygonal ferrite-pearlite steel having predetermined phase fractions can be hardly obtained, and the cold workability or surface quality after working is deteriorated. On the other hand, if the rapid cooling stop temperature is 650° C. or more, the precipitated carbide in ferrite is coarsened, and the fatigue strength is reduced. The rapid cooling stop temperature is preferably from 560 to 640° C. and more preferably from 580 to 620° C.

[Slow Cooling After Stopping of Rapid Cooling]

After stopping the rapid cooling, the sheet is slowly cooled by standing to cool or air cooling at a cooling rate (slow cooling rate) of 10° C./s or less for 5 to 20 seconds. Accordingly, the precipitated carbide in ferrite is appropriately refined while allowing polygonal ferrite formation to proceed sufficiently. If the cooling rate exceeds 10° C./s or the slow cooling time is less than 5 seconds, the amount of polygonal ferrite formed is insufficient, whereas if the slow cooling time exceeds 20 seconds, the precipitated carbide is not coarsened and the fatigue strength is deteriorated.

[Rapid Cooling and Coiling After Slow Cooling]

After the slow cooling, the sheet is again rapidly cooled at a cooling rate (second rapid cooling rate) of 20° C./s or more and coiled at 500 to 600° C. This is performed so as to form a bainitic ferrite+ferrite-based microstructure and thus ensure cold workability. If the cooling rate (second rapid cooling rate) is less than 20° C./s or the coiling temperature exceeds 600° C., the cold workability is deteriorated due to formation of a large amount of pearlite, whereas if it is less than 500° C., the amount of bainitic ferrite formed is insufficient and the surface quality after working is deteriorated.

The present invention is described in greater detail below by referring to

Examples, but the present invention is by no means limited to the following Examples and may be carried out by appropriately making changes as long as they are in conformity to the gist described hereinabove and hereinafter, all of which are included in the technical scope of the present invention.

EXAMPLES

Steel having a component composition shown in Table 1 below was melted by a vacuum melting method and cast into an ingot having a thickness of 120 mm, which was subjected to hot rolling under conditions shown in Table 2 below to produce a hot-rolled steel sheet. In each test, the cooling rate until stopping of rapid cooling after the completion of finish rolling was 20° C./s or more, and the cooling after stopping the rapid cooling had the conditions in which a slow cooling is performed at a cooling rate of 10° C./s or less for from 5 to 20 s.

The thus-obtained hot-rolled steel sheets were measured for the solute N content, area ratio of each phase in the steel sheet microstructure, and average grain size of bainitic ferrite, by respective measurement methods described in “MODE FOR CARRYING OUT THE INVENTION” above.

In addition, those hot-rolled steel sheets were evaluated as follows for the cold workability as well as the surface quality and hardness after working.

(Evaluation of Cold Workability)

The cold workability was evaluated based on the amount of work hardening defined by the difference in the hardness's at the sample central part between before and after working. First, the hot-rolled steel sheet was subjected to a working test by using a working reproducing tester (manufactured by Fuji Electronic Industrial Co., Ltd, Thermecmaster Z) under the conditions of working temperature: room temperature, rolling reduction: 70% and strain rate: 10/s. The shape of the working specimen was adjusted according to the sheet thickness such that the diameter/height ratio became substantially constant, e.g., φ8 mm×12 mm, φ6 mm×10 mm, or φ4 mm×6 mm. In the measurement of hardness, the Vickers hardness (Hv) of each working specimen was measured before and after the working test above by using a Vickers hardness tester and setting the measurement position at ¼ portion (position in the midst between the center and the outer circumference) in the circle diameter direction at the central part in the compression direction of the working specimen, under the conditions of load: 500 g and number of measurements: 5, and respective averages were designated as the pre-working hardness and the post-working hardness.

Then, the cold workability was evaluated, as described above, based on the amount of work hardening defined by “post-working hardness-pre-working hardness”. A larger amount of work hardening means better workability, and 80 Hv or more was judged as passed.

(Evaluation of Hardness After Working)

As the evaluation of hardness after working, evaluation was performed based on the above-described post-working hardness measured after the working test and sample specimen of 250 Hv or more was judged as passed.

(Evaluation of Surface Quality After Working)

The surface quality after working was evaluated by the presence or absence of SS mark generation after a tensile test. For this purpose, a No. 5 specimen (25 mm×50 mm×[from sheet surface to center in the sheet thickness direction (the sheet thickness was adjusted by taking into account the capability of the tensile tester)]) of JIS Z 2201 in a direction perpendicular to the rolling direction was sampled and subjected to a tensile test based on JIS Z 2241 (1980) (Method for Tensile Test of Metal Material) under the conditions of room temperature of 20° C. and an initial crosshead speed of 600 mm/min until the strain amount reached 15%. The presence or absence of SS mark generation on the surface of the specimen after the tensile test was made to appear clear by grinding the specimen surface with a grindstone and evaluated with an eye. The sample specimen was judged as passed when SS mark was not generated, and judged as failed when SS mark was generated.

These measurement results are shown in Table 3.

TABLE 1 Components (mass %) Steel [remainder: Fe and unavoidable impurities] Species C Si Mn P S Al N 10C + N Others a 0.05 0.02 0.40 0.007 0.001 0.025 0.011 0.51 — b 0.08 0.02 0.40 0.007 0.001 0.022 0.008 0.81 — c 0.08 0.02 0.40 0.007 0.001 0.022 0.023 0.82 — d 0.08 0.10 0.30 0.007 0.001 0.023 0.009 0.89 — e 0.08 0.40 0.20 0.007 0.001 0.024 0.009 0.89 — f 0.11 0.02 0.40 0.007 0.001 0.022 0.010 1.11 — g 0.15 0.02 0.40 0.007 0.001 0.024 0.009 1.51 — h 0.20 0.02 0.40 0.007 0.001 0.022 0.010 2.01 — i 0.26 0.02 0.40 0.007 0.001 0.023 0.009 2.61 — j 0.08 0.02 0.40 0.007 0.001 0.025 0.003 0.80 — k 0.08 0.02 0.40 0.007 0.001 0.025 0.030 0.83 — l 0.31 0.02 0.40 0.007 0.001 0.025 0.008 3.11 — m 0.08 0.60 0.40 0.007 0.001 0.025 0.010 0.81 — n 0.08 0.02 0.15 0.007 0.001 0.025 0.012 0.81 — o 0.08 0.02 1.10 0.007 0.001 0.025 0.011 0.81 — p 0.08 0.02 0.40 0.060 0.001 0.025 0.010 0.81 — q 0.08 0.02 0.40 0.007 0.060 0.025 0.011 0.81 — r 0.08 0.02 0.40 0.007 0.001 0.005 0.012 0.81 — s 0.08 0.02 0.40 0.007 0.001 0.11  0.013 0.81 — t 0.08 0.02 0.40 0.007 0.001 0.025 0.010 0.81 Cr: 0.5, Nb: 0.03 u 0.08 0.02 0.40 0.007 0.001 0.025 0.010 0.81 Cu: 0.06, Ni: 0.15 v 0.08 0.02 0.40 0.007 0.001 0.025 0.009 0.81 Ca: 0.0025, Li: 0.001 w 0.08 0.02 0.40 0.007 0.001 0.025 0.009 0.81 Cr: 0.5, Nb: 0.03 x 0.30 0.02 0.40 0.007 0.001 0.024 0.025 3.03 — (—: not added, underlined: outside the scope of the present invention)

TABLE 2 Hot Rolling Conditions Rapid Final Finish Cooling Thickness Heating Rolling Rolling Stop Coiling of Hot- Production Steel Temperature Reduction Temperature Temperature Temperature Rolled No. Species (° C.) (%) (° C.) (° C.) (° C.) Sheet (mm)  1 a 1250 15 902 627 557 10  2 a 1250 17 908 621 595  4  3 a 1250 15 893 620 545 18  4* a  1000*  14*  803*  526*  412* 10  5* a 1250  14* 905 571 521 25  6* a 1250  9* 899 579 552 10  7 b 1250 17 920 581 514 10  8 c 1250 15 921 609 531 10  9 d 1250 15 915 616 532 10 10 e 1250 15 916 611 532 10 11 f 1250 16 907 587 542 10 12 g 1250 15 906 576 511 10 13 h 1250 16 901 570 520 10 14 i 1250 17 909 634 559 10 15 j 1250 15 912 610 532 10 16 k 1250 16 910 581 533 10 17 l 1250 16 926 639 549 10 18 m 1250 16 889 569 501 10 19 n 1250 16 906 574 523 10 20 o 1250 15 893 570 568 10 21 p 1250 16 919 595 550 10 22 q 1250 15 920 591 521 10 23 r 1250 16 922 599 595 10 24 s 1250  14* 890 596 579 10 25 t 1250 16 892 571 560 10 26 u 1250 15 895 625 591 10 27 v 1250 15 896 564 518 10 28 w 1250 16 911 601 570 10 29 x 1250 16 906 590 507 10 (underlined: outside the scope of the present invention, *outside the recommended range)

TABLE 3 Presence or Post- Amount of Solute N Microstructure Absence of Working Work Steel Steel Production Amount Area Ratio (%) BF Average SS Mark Hardness Hardening No. Species No. (mass %) BF PF P Grain Size (μm) Generation (Hv) (Hv) Remarks 1 a  1  0.0084 10 87 3 27 none 257  93 steel of invention 2 a  2 0.008 25 68 7 14 none 273 101 steel of invention 3 a  3 0.008  8 89 3 34 none 251  83 steel of invention 4 a  4* 0.003 69  8 23  21 none 192  53 comparative steel 5 a  5* 0.009  3 92 5 53 generated 193  65 comparative steel 6 a  6* 0.008  4 88 8 51 generated 195  71 comparative steel 7 b  7 0.007 45 51 4 23 none 290 103 steel of invention 8 c  8 0.019 48 47 5 22 none 295 107 steel of invention 9 d  9  0.0085 53 43 4 20 none 303 105 steel of invention 10 e 10 0.008 58 38 4 19 none 311 106 steel of invention 11 f 11 0.009 65 30 5 16 none 307 111 steel of invention 12 g 12 0.008 71 18 11  11 none 322 103 steel of invention 13 h 13 0.009 77 10 13  10 none 335 102 steel of invention 14 i 14 0.008 83  2 15   8 none 353  91 steel of invention 15 j 15 0.002 46 50 4 24 none 171  71 comparative steel 16 k 16 0.025 49 48 3 22 generated — — comparative steel 17 l 17 0.007 43 32 25   9 generated — — comparative steel 18 m 18  0.0085 63 32 5 23 none — — comparative steel 19 n 19 0.010 22 74 4 22 none 221  71 comparative steel 20 o 20 0.009 43 51 6 21 none — — comparative steel 21 p 21 0.009 41 54 5 26 none — — comparative steel 22 q 22 0.010 45 51 4 27 none — — comparative steel 23 r 23 0.011 39 57 4 26 none — — comparative steel 24 s 24 0.012 55 41 4 24 none — — comparative steel 25 t 25 0.008 55 42 3 15 none 290 103 steel of invention 26 u 26 0.009 58 38 4 16 none 280 105 steel of invention 27 v 27 0.008 49 47 4 17 none 270 104 steel of invention 28 w 28 0.008 63 34 3 15 none 275  98 steel of invention 29 x 29 0.020 75  6 19  10 generated — — comparative steel (underlined: outside the scope of the present invention, *outside the recommended range, BF: bainitic ferrite, PF: polygonal ferrite, P: pearlite, —: not measured because cracking was generated before reaching the predetermined rolling reduction during working test, steel of invention: [no SS mark generation] and [post-working hardness after working ≧250 Hv] and [amount of work hardening ≧80 Hv], comparative steel: when the above-described conditions of the steel of invention are not satisfied)

As shown in Table 3, each of Steel Nos. 1 to 3, 7 to 14 and 25 to 28 was produced by using a steel species satisfying the requirements specified for the component composition of the present invention under the recommended hot rolling conditions, and as a result, it could be confirmed that these are steel of the invention fulfilling the requirements specified for the microstructure of the present invention, all of the surface property after working, the post-working hardness and the amount of work hardening meet the acceptance standards, and a hot-rolled steel sheet exhibiting good cold workability during working and exhibiting predetermined surface quality and hardness (strength) after working can be obtained.

On the other hand, Steel Nos. 4 to 6, 15 to 24 and 29 are comparative steels failing in satisfying at least one of the requirements specified for the component composition and the microstructure in the present invention, where at least one of the surface property after working, the post-working hardness and therefore the amount of work hardening does not meet the acceptance standard.

For example, Steel No. 4 satisfies the requirements for the component composition, but since the heating temperature before hot rolling is outside the recommended range and is too low, the solute N amount is insufficient and therefore the post-working hardness is inferior.

Steel No. 5 satisfies the requirements for the component composition, but since the sheet thickness after hot rolling is outside the specified range and is too large, the bainitic ferrite is lacking but, on the other hand, is coarsened, and therefore both the post-working hardness and the amount of work hardening are inferior.

Steel No. 6 satisfies the requirements for the component composition, but since the final rolling reduction at the time of hot rolling is outside the recommended range and is too small, the bainitic ferrite is lacking but, on the other hand, is coarsened, and therefore both the post-working hardness and the amount of work hardening are inferior.

In Steel No. 15 (steel species j) where the hot rolling conditions are in the recommended range but the N content is too low, both the post-working hardness and the amount of work hardening are inferior.

On the other hand, in Steel No. 16 (steel species k) where the hot rolling conditions are in the recommended range but the N content is too high, not only the cold workability but also the surface property after working is inferior.

In Steel No. 17 (steel species 1) where the hot rolling conditions are in the recommended range but the C content is too high and the requirement of 10C+N≦3 0 is not satisfied, pearlite is excessively formed and not only the cold workability but also the surface property after working is inferior.

In Steel No. 18 (steel species m) where the hot rolling conditions are in the recommended range but the Si content is too high, at least the cold workability is inferior.

In Steel No. 19 (steel species n) where the hot rolling conditions are in the recommended range but the Mn content is too low, both the post-working hardness and the amount of work hardening are inferior.

On the other hand, in Steel No. 20 (steel species o) where the hot rolling conditions are in the recommended range but the Mn content is too high, at least the cold workability is inferior.

In Steel No. 21 (steel species p) where the hot rolling conditions are in the recommended range but the P content is too high, at least the cold workability is inferior.

In Steel No. 22 (steel species q) where the hot rolling conditions are in the recommended range but the S content is too high, at least the cold workability is inferior.

In Steel No. 23 (steel species r) where the hot rolling conditions are in the recommended range but the Al content is too low, at least the cold workability is inferior.

In Steel No. 24 (steel species s) where the hot rolling conditions except for the final rolling reduction are in the recommended range but the Al content is too high, at least the cold workability is inferior.

On the other hand, in Steel No. 29 (steel species x) where the hot rolling conditions are in the recommended range but the requirement of 10C-FN≦3.0 is not satisfied, not only the cold workability but also the surface property after working are inferior.

From the above, the applicability of the present invention could be confirmed.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the present invention.

This application is based on Japanese Patent Application (Patent Application No. 2013-183091) filed on Sep. 4, 2013, the contents of which are incorporated herein by way of reference.

INDUSTRIAL APPLICABILITY

The hot-rolled steel material of the present invention is useful, e.g., for various parts for automotive (for example, transmission parts such as gear, and casings) and can realize lighter weight and higher strength. 

1. A hot-rolled steel sheet excellent in cold workability as well as in surface property and hardness after working, wherein: a sheet thickness is from 3 to 20 mm; a component composition comprises, in mass % (hereinafter, the same applies to chemical components), C: 0.3% or less (exclusive of 0%), Si: 0.5% or less (exclusive of 0%), Mn: from 0.2 to 1%, P: 0.05% or less (exclusive of 0%), S: 0.05% or less (exclusive of 0%), Al: from 0.01 to 0.1%, and N: from 0.008 to 0.025%, with a remainder being iron and unavoidable impurities, wherein solute N: 0.007% or more and the contents of C and N satisfy the relationship of 10C+N≦3.0; a microstructure comprises, in terms of area ratio relative to an entire microstructure, bainitic ferrite: 5% or more, pearlite: less than 20%, and remainder: polygonal ferrite; and an average grain size of the bainitic ferrite is in a range of from 3 to 50 μm.
 2. The hot-rolled steel sheet according to claim 1, wherein the component composition further comprises at least one member of the following (a) to (e): (a) at least one of Cr: 2% or less (exclusive of 0%) and Mo: 2% or less (exclusive of 0%), (b) at least one member selected from the group consisting of Ti: 0.2% or less (exclusive of 0%), Nb: 0.2% or less (exclusive of 0%) and V: 0.2% or less (exclusive of 0%), (c) B: 0.005% or less (exclusive of 0%), (d) at least one member selected from the group consisting of Cu: 5% or less (exclusive of 0%), Ni: 5% or less (exclusive of 0%) and Co: 5% or less (exclusive of 0%), and (e) at least one member selected from the group consisting of Ca: 0.05% or less (exclusive of 0%), REM: 0.05% or less (exclusive of 0%), Mg: 0.02% or less (exclusive of 0%), Li: 0.02% or less (exclusive of 0%), Pb: 0.5% or less (exclusive of 0%), and Bi: 0.5% or less (exclusive of 0%). 